Field-oriented control for an AC motor drive is well known. Based on a reference frame which rotates at the speed of the rotor flux, a flux component and a torque component of the stator currents oriented upon such reference frame are calculated and used to control the motor flux and the resulting torque. See for instance FIELD-ORIENTED CONTROL OF A STANDARD AC MOTOR USING MICROPROCESSORS by R. Gabriel, W. Leonard and C. J. Nordby, IEEE Trans. IA-16, pages 186-192, March/April 1980; INTRODUCTION TO FIELD ORIENTATION AND HIGH PERFORMANCE AC DRIVES by D. W. Novotny and R. D. Lorenz, IEEE Industry Applications Society, October 6-6, 1985, Toronto, Canada, Section 2, pages 2-1 to 2-65. The two afore-cited publications are hereby incorporated by reference. The afore-mentioned W. Leonard and the D. W. Novotny and R. D. Lorenz publications are hereby incorporated by reference.
The assumption is that the motor flux .psi.* and T* demand signals can be instantaneously satisfied under the further assumption that the mathematical model used is accurate, that the parameter T2, namely the rotor time constant, is known and that the specified direct and quadrature current components i.sub.d and i.sub.q can be instantaneously injected into the stator winding.
Direct and quadrature stator currents have been generated for control according to the vector control method described in U.S. Pat. No. 4,456,868 of Yamamura et al. The purpose, there, is to improve the response on the torque.
It is also known from U.S. Pat. No. 4,125,796 of Nagase et al. to generate a desired torque by calculating a current pattern signal, also by decomposing the motor current into a flux oriented direction and in quadrature thereto.
U.S. Pat. No. 4,451,771 of Nagase et al. discloses the generation of a current correction signal applied to the current control signal derived according to the motor control method in an AC motor drive.
The object of the present invention is to achieve a speed regulator providing dynamic control of both the motor speed and the magnetic flux level, thereby to ensure that control is maintained over the field-weakening operative range of the motor drive.
The present invention involves a speed regulator system wherein both the torque and flux references are variables. The torque demand is derived from the speed regulator error signal and the motor flux reference is a predefined function of the motor speed.
As long as in the motor drive, the flux is held constant, or merely gradually changing, the prior art technique of vector control can accommodate speed regulation. If, however, the speed is called to accelerate rapidly, or conversely, to decelerate rapidly, the problem arises of dynamically forcing the flux to match such circumstance. Since there are two variable current components, the problem translates itself into how to selectively exercise the compensating effect on those two components so as to cause the resultant vector to match the speed requirements. The major obstacle with such rapidly changing demand is to prevent the current from exceeding acceptable limits. Therefore, the question arises as to how the total current should be limited to a safe maximum value. Imposing constant limits on both components would unnecessarily restrict one component in magnitude whenever the demand for the other is low.